Curable fluoroelastomer composition

ABSTRACT

Fluoroelastomer compositions comprising fluoroelastomers having copolymerized units of a nitrile-containing cure site monomer are cured with certain hydrazide curatives. The hydrazide is of the general formula R 1 (C(O)) n NHNHR 2 , wherein n is 1 or 2; R 1  is NH 2 , NHNH 2 , NHR 3 , NR 3   2 , NHNHC(O)NH 2 , NHNHC(O)NHNH 2 , NHNHR 2 , or NHC(O)NHNH 2 ; R 2  is H, alkyl, aryl, heterocycle, CO 2 R 3 , C(O)R 3 , CH 2 R 4 , or C(O)R 4 ; R 3  is alkyl, aryl, heterocycle, or CH 2 R 4 ; and R 4  is a fluoroalkyl group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/469,856 filed Mar. 31, 2011.

FIELD OF THE INVENTION

This invention relates to curable fluoroelastomer compositions and moreparticularly to fluoroelastomer compositions containing certainhydrazides as curing agents.

BACKGROUND OF THE INVENTION

Fluoroelastomers have achieved outstanding commercial success and areused in a wide variety of applications in which severe environments areencountered, in particular those end uses where exposure to hightemperatures and aggressive chemicals occurs. For example, thesepolymers are often used in seals for aircraft engines, in oil-welldrilling devices, and in sealing elements for industrial equipment thatoperates at high temperatures.

The outstanding properties of fluoroelastomers are largely attributableto the stability and inertness of the copolymerized fluorinated monomerunits that make up the major portion of the polymer backbones in thesecompositions. Such monomers include vinylidene fluoride,hexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinyl)ethers. In order to develop elastomeric properties fully,fluoroelastomers are typically crosslinked, i.e. vulcanized. To thisend, a small percentage of cure site monomer is copolymerized with thefluorinated monomer units. Cure site monomers containing at least onenitrile group, for exampleperfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred.Such compositions are described in U.S. Pat. Nos. 4,281,092; 4,394,489;5,789,489; 5,789,509 and in WO 2011084404.

Bisamidoximes (U.S. Pat. No. 5,668,221) and bisamidrazones (U.S. Pat.Nos. 5,605,973; 5,637,648) have been used as vulcanizing agents forfluoroelastomers having nitrile group cure sites. These cures may bescorchy, i.e. crosslinking may begin before the final shaping of thecomposition. Also, the curatives require complex, multistep synthesesfrom expensive starting materials.

Other nitrogen containing nucleophilic compounds have been employed tocrosslink fluoroelastomers having nitrile group cure sites (U.S. Pat.No. 6,638,999 B2). Some of these curatives are scorchy while others arevolatile at rubber milling temperatures.

Fluoropolymers having pendant amidrazone or amidoxime groups are alsoknown (U.S. Pat. No. 7,300,985 B2). These polymers require an additionalpolymer modification step in order to form crosslinks.

SUMMARY OF THE INVENTION

The present invention is directed to curable fluoroelastomercompositions which comprise a fluoroelastomer having nitrile group curesites and certain hydrazide as curatives. More specifically, the presentinvention is directed to a curable composition comprising:

-   -   A) a fluoroelastomer comprising copolymerized units of a nitrile        group-containing cure site monomer; and    -   B) a hydrazide of the general formula R¹(C(O))_(n)NHNHR²,        wherein n is 1 or 2; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHC(O)NH₂,        NHNHC(O)NHNH₂, NHNHR₂, or NHC(O)NHNH₂; R² is H, alkyl, aryl,        heterocycle, CO₂R³, C(O)R³, CH₂R⁴, or C(O)R⁴; R³ is alkyl, aryl,        heterocycle, or CH₂R⁴; and R⁴ is a fluoroalkyl group.

Another aspect of the present invention is a cured article made from theabove composition.

DETAILED DESCRIPTION OF THE INVENTION

The fluoroelastomer that may be employed in the composition of theinvention may be partially fluorinated or perfluorinated.Fluoroelastomers preferably contain between 25 and 70 weight percent,based on the total weight of the fluoroelastomer, of copolymerized unitsof a first monomer which may be vinylidene fluoride (VF₂) ortetrafluoroethylene (TFE). The remaining units in the fluoroelastomersare comprised of one or more additional copolymerized monomers,different from said first monomer, selected from the group consisting offluoromonomers, hydrocarbon olefins and mixtures thereof. Fluoromonomersinclude fluorine-containing olefins and fluorine-containing vinylethers.

Fluorine-containing olefins which may be employed to makefluoroelastomers include, but are not limited to vinylidene fluoride(VF₂), hexafluoropropylene (HFP), tetrafluoroethylene (TFE),1,2,3,3,3-pentafluoropropene (1-HPFP), 1,1,3,3,3-pentafluoropropene(2-HPFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.

Fluorine-containing vinyl ethers that may be employed to makefluoroelastomers include, but are not limited to perfluoro(alkyl vinyl)ethers. Perfluoro(alkyl vinyl) ethers (PAVE) suitable for use asmonomers include those of the formulaCF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)  (I)where R_(f′), and R_(f″) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl) ethers includes compositionsof the formulaCF₂═CFO(CF₂CFXO)_(n)R_(f)  (II)where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms.

A most preferred class of perfluoro(alkyl vinyl) ethers includes thoseethers wherein n is 0 or 1 and R_(f) contains 1-3 carbon atoms. Examplesof such perfluorinated ethers include perfluoro(methyl vinyl ether)(PMVE), perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinylether) (PPVE). Other useful monomers include those of the formulaCF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)  (III)where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z=F or CF₃. Preferred members of this class are those inwhich R_(f) is C₃F₇, m=0, and n=1.

Additional perfluoro(alkyl vinyl) ether monomers include compounds ofthe formulaCF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (IV)where m and n independently=0-10, p=0-3, and x=1-5. Preferred members ofthis class include compounds where n=0-1, m=0-1, and x=1.

Other examples of useful perfluoro(alkyl vinyl ethers) includeCF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)  (V)where n=1-5, m=1-3, and where, preferably, n=1.

If copolymerized units of PAVE are present in fluoroelastomers employedin the invention, the PAVE content generally ranges from 25 to 75 weightpercent, based on the total weight of the fluoroelastomer. Ifperfluoro(methyl vinyl ether) is used, then the fluoroelastomerpreferably contains between 30 and 65 wt. % copolymerized PMVE units.

Hydrocarbon olefins useful in the fluoroelastomers employed in theinvention include, but are not limited to ethylene and propylene. Ifcopolymerized units of a hydrocarbon olefin are present in thefluoroelastomers, hydrocarbon olefin content is generally 4 to 30 weightpercent.

The fluoroelastomer further contains copolymerized units of at least onecure site monomer, generally in amounts of from 0.1-5 mole percent. Therange is preferably between 0.3-1.5 mole percent. Although more than onetype of cure site monomer may be present, most commonly one cure sitemonomer is used and it contains at least one nitrile substituent group.Suitable cure site monomers include nitrile-containing fluorinatedolefins and nitrile-containing fluorinated vinyl ethers. Usefulnitrile-containing cure site monomers include those of the formulasshown below.CF₂═CF—O(CF₂)_(n)—CN  (VI)

where n=2-12, preferably 2-6;CF₂═CF—O[CF₂—CFCF₃—O]_(n)—CF₂—CFCF₃—CN  (VII)

where n=0-4, preferably 0-2;CF₂═CF—[OCF₂CFCF₃]_(x)—O—(CF₂)_(n)—CN  (VIII)

where x=1-2, and n=1-4; andCF₂═CF—O—(CF₂)_(n)—O—CF(CF₃)CN  (IX)

where n=2-4.

Those of formula (VIII) are preferred. Especially preferred cure sitemonomers are perfluorinated polyethers having a nitrile group and atrifluorovinyl ether group. A most preferred cure site monomer isCF₂═CFOCF₂CF(CF₃)OCF₂CF₂CN  (X)i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.

A first aspect of this invention is a curable composition comprising A)a fluoroelastomer comprising copolymerized units of a cure site monomerselected from the group consisting of nitrile-containing fluorinatedolefins and nitrile-containing fluorinated vinyl ethers; and B) acertain hydrazide. The hydrazide may be a mono-, di-, or poly-hydrazide.The hydrazide is of the general formula R¹ (C(O))_(n)NHNHR², wherein nis 1 or 2; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHC(O)NH₂, NHNHC(O)NHNH₂,NHNHR², or NHC(O)NHNH₂; R² is H, alkyl, aryl, heterocycle, CO₂R³,C(O)R³, CH₂R⁴, or C(O)R⁴; R³ is alkyl, aryl, heterocycle, or CH₂R⁴; andR⁴ is a fluoroalkyl group. The alkyl, aryl, benzyl or heterocycle groupsmay contain additional functional groups such as, but not limited tohalogen, ether, or amide groups. The fluoroalkyl group has at least oneof the hydrogen atoms replaced by fluorine and may optionally contain 1or more oxygen atoms in the chain. R², R³, or R⁴ may have 1, 2, or morepoints of attachment to a hydrazide to afford mono-, di-, orpoly-hydrazide. Compounds that decompose to form one of these hydrazidesmay also be employed in the composition of the invention. The hydrazidemay also be a salt, e.g. semicarbazide hydrochloride. Lastly, the oxygenatom may be replaced by sulfur.

Preferred hydrazides include those wherein n is 1; R¹ is NH₂, NHNH₂,NHR³, NR³ ₂, NHNHC(O)NH₂, NHNHC(O)NHNH₂, NHNHR², or NHC(O)NHNH₂; and R²is H; n is 2; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHC(O)NH₂, NHNHC(O)NHNH₂,NHNHR², or NHC(O)NHNH₂; and R² is H; n is 1; R¹ is NH₂, NHNH₂, NHR³, NR³₂, NHNHC(O)NH₂, NHNHC(O)NHNH₂, NHNHR², or NHC(O)NHNH₂; and R² is CO₂R³;and n is 2; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHC(O)NH₂, NHNHC(O)NHNH₂,NHNHR², or NHC(O)NHNH₂; and R² is CO₂R³.

Specific examples of these hydrazide curatives include, but are notlimited to carbohydrazide; carbonic dihydrazide, dihydrochloride;4-phenylsemicarbazide; 4-phenylsemicarbazide hydrochloride;4,4-diphenylsemicarbazide; oxamic hydrazide; oxalyldihydrazide;semicarbazide; 1,2-hydrazinedicarboxylic acid, dihydrazide;diaminobiuret; thiocarbohydrazide; and thiosemicarbazide. An example ofa dihydrazide is p-phenylenebissemicarbazide. More preferred hydrazidesinclude carbohydrazide; 4-phenylsemicarbazide; oxamic hydrazide;semicarbazide, and oxalyldihydrazide.

It is theorized that these hydrazides act as curing agents by causingthe dimerization of polymer chain bound nitrile groups to form1,2,4-triazole rings, thus crosslinking the fluoroelastomer. Thehydrazides are less volatile than curatives such as hydrazine or t-butylcarbazate, making the hydrazide curatives less likely to be fugitiveduring mixing and shaping processes.

In order to be useful as either the major, or as the only curative forthese fluoroelastomers, the level of hydrazide should be about 0.05 to 7parts hydrazide per 100 parts fluoroelastomer, preferably about 0.1 to 3parts hydrazide per 100 parts fluoroelastomer, most preferably about 0.5to 2 parts hydrazide per 100 parts fluoroelastomer. As used herein,“parts” refers to parts by weight, unless otherwise indicated.

An appropriate level of hydrazide can be selected by considering cureproperties, for example the time to develop maximum moving die rheometer(MDR) torque and minimum Mooney scorch of the curable compositions. Theoptimum level will depend on the particular combination offluoroelastomer and hydrazide.

Optionally, a curative accelerator, e.g. a compound that releasesammonia at curing temperatures, may be used in combination with ahydrazide curative. Examples of compounds that decompose to releaseammonia at curing temperatures include those disclosed in U.S. Pat. No.6,281,296 B1 and U.S. 2011/0009569.

Optionally, another curative commonly employed to crosslinkfluoroelastomers having nitrile-group cure sites may be used in additionto the hydrazide. Examples of such other curatives include, but are notlimited to diaminobisphenol AF,2,2-bis(3-amino-4-anilinophenyl)hexafluoropropane, mono- orbis-amidines, mono- or bis-amidrazones, mono- or bis-amidoximes, or anorganic peroxide plus coagent.

Additives, such as carbon black, fluoropolymer micropowders,stabilizers, plasticizers, lubricants, fillers, and processing aidstypically utilized in fluoroelastomer compounding can be incorporatedinto the compositions of the present invention, provided they haveadequate stability for the intended service conditions.

The curable compositions of the invention may be prepared by mixing thefluoroelastomer, hydrazide and other components using standard rubbercompounding procedures. For example, the components may be mixed on atwo roll rubber mill, in an internal mixer (e.g. a Banbury® internalmixer), or in an extruder. The curable compositions may then becrosslinked (i.e. cured) by application of heat and/or pressure. Whencompression molding is utilized, a press cure cycle is generallyfollowed by a post cure cycle during which the press cured compositionis heated at elevated temperatures in excess of 300° C. for severalhours.

The curable compositions of the present invention are useful inproduction of gaskets, tubing, and seals. Such cured articles aregenerally produced by molding a compounded formulation of the curablecomposition with various additives under pressure, curing the part, andthen subjecting it to a post cure cycle. The cured compositions haveexcellent thermal stability, steam and chemical resistance. Volume swell(ASTM D1414) after exposure to 225° C. water for at least 168 hours,preferably at least 336 hours, is less than 5%. Also compression set,300° C., 70 hours, 15% compression (ASTM D395) is less than 70%. Thecured compositions are particularly useful in applications such as sealsand gaskets for manufacturing semiconductor devices, and in seals forhigh temperature automotive uses.

Other fluoropolymers containing nitrile cure sites, such asfluoroplastics may be substituted for fluoroelastomers in thecompositions of the invention.

The invention is now illustrated by certain embodiments wherein allparts are by weight unless otherwise specified.

EXAMPLES Test Methods

Cure Characteristics

Cure characteristics were measured using a Monsanto MDR 2000 instrumentunder the following conditions:

-   -   Moving die frequency: 1.66 Hz    -   Oscillation amplitude: ±0.5 degrees    -   Temperature: 190° C., unless otherwise noted    -   Sample size: Disks having diameter of 1.5 inches (38 mm)    -   Duration of test: 30 minutes

The following cure parameters were recorded:

-   -   M_(H): maximum torque level, in units of dN·m    -   M_(L): minimum torque level, in units of dN·m    -   Tc90: time to 90% of maximum torque, minutes

Test specimens were prepared from elastomer compounded with appropriateadditives, as described in the formulations listed in the Examplesbelow. Compounding was carried out on a rubber mill. The milledcomposition was formed into a sheet and a 10 g sample was die cut into adisk to form the test specimen.

Compression set of O-ring samples was determined in accordance with ASTMD395. Mean values are reported.

Volume swell in water was measured at 225° C. for the time indicated inthe Tables in accordance with ASTM D1414.

The following fluoroelastomer polymers were used in the Examples:

-   FFKM—A terpolymer containing 61.8 mole percent units of TFE, 37.4    mole percent units of PMVE and 0.80 mole percent units of 8-CNVE was    prepared according to the general process described in U.S. Pat. No.    5,789,489.-   FKM—A terpolymer containing 23.3 mole percent units of TFE, 50.7    mole percent units of VF₂, 25.1 mole percent units of    hexafluoropropylene and 0.90 mole percent units of 8-CNVE.

Examples 1-2

Curable compositions of the invention were compounded on a two-rollrubber mill in the proportions shown in Table I. The compoundedcompositions are labeled Example 1 (4-phenylsemicarbazide, availablefrom Alfa-Aesar) and Example 2 (carbohydrazide, available fromSigma-Aldrich) in Table I. Cure characteristics of the compoundedcompositions are also shown in Table I. O-rings were made by presscuring the curable compositions at a temperature of 190° C. for Tc90plus 5 minutes, followed by a post cure in a nitrogen atmosphere at atemperature of 305° C. for 26 hours after a slow temperature ramp upfrom room temperature. Compression set and volume swell values are alsoshown in Table I.

In order to compare volume swells of the compositions of the inventionwith a prior art composition, o-rings were made from a similar compound,but containing 0.25 phr urea as curative, rather than a hydrazidecurative. After only 168 hours of exposure to 225° C. water, the ureacured o-rings exhibited a 15.7% volume swell.

TABLE I Example Example l 2 Formulation (phr¹) FFKM 100 100 Carbon BlackMT 30 30 N990 4-phenyl- 1.14 0 semicarbazide Carbohydrazide 0 0.68 CureCharacteristics M_(L) (dN · m) 1.97 3.23 M_(H) (dN · m) 14.06 17.07Tc90, minutes 16.5 18.5 Compression set, 41.3 27.0 300° C., 70 hours,15% compression, % Volume swell, 0.6 1.2 336 hours, % ¹Parts by weightper hundred parts by weight fluoroelastomer

Examples 3-5

Curable compositions of the invention were compounded on a two-rollrubber mill in the proportions shown in Table II. The compoundedcompositions are labeled Example 3 (oxamic hydrazide, available fromAcros Organics), Example 4 (oxalyl dihydrazide, available fromSigma-Aldrich) and Example 5 (semicarbazide, prepared from urea andhydrazine hydrate according to the general procedure in Example 1 ofU.S. Pat. No. 4,482,738, except hydrogen chloride was omitted) in TableII. Cure characteristics of the compounded compositions are also shownin Table II.

TABLE II Example Example Example 3 4 5 Formulation (phr) FFKM 100 100100 Carbon Black MT 30 30 30 N990 Oxamic 0.78 0 0 hydrazide Oxalyl 00.89 0 dihydrazide Semicarbazide 0 0 0.57 Cure Characteristics M_(L) (dN· m) 2.38 2.95 3.55 M_(H) (dN · m) 17.30 18.03 18.74 Tc90, minutes 118.3 9.0 Compression set, 33.3 25.4 25.4 300° C., 70 hours, 15%compression, % Volume swell, 1.6 1.2 0.5 168 hours, %

Examples 6-8

Curable compositions of the invention were compounded on a two-rollrubber mill in the proportions shown in Table III. The compoundedcompositions are labeled Example 6 (4-phenylsemicarbazide hydrochloride,available from Sigma-Aldrich), Example 7 (4,4-diphenylsemicarbazide,prepared from diphenylcarbamoyl chloride and anhydrous hydrazine) andExample 8 (semicarbazide hydrochloride, available from Sigma-Aldrich) inTable III. Cure characteristics of the compounded compositions are alsoshown in Table III.

TABLE III Example Example Example 6 7 8 Formulation (phr) FFKM 100 100100 Carbon Black MT 30 30 30 N990 4-phenyl- 1.42 0 0 semicarbazidehydrochloride 4,4- 0 1.72 0 diphenylsemicarb azide Semicarbazide 0 00.84 hydrochloride Cure Characteristics M_(L) (dN · m) 1.75 1.71 2.22M_(H) (dN · m) 6.69 3.04 6.01 Tc90, minutes 26.8 26.0 27.0 Compressionset, 47.6 44.4 36.5 300° C., 70 hours, 15% compression, % Volume swell,1.7 2.0 2.0 168 hours, %

Example 9

A curable composition of the invention was compounded on a two-rollrubber mill in the proportions shown in Table IV. The compoundedcomposition is labeled Example 9 (carbohydrazide) in Table IV. Curecharacteristics (at 199° C.) of the compounded compositions are alsoshown in Table IV. O-rings were made by press curing the curablecomposition at a temperature of 199° C. for Tc90 plus 5 minutes,followed by a post cure in a nitrogen atmosphere at a temperature of260° C. for 8 hours after a slow temperature ramp up from roomtemperature. Compression set is also shown in Table IV.

TABLE IV Example 9 Formulation (phr) FKM 100 Carbon Black MT 30 N990Carbohydrazide 1.0 Cure Characteristics M_(L) (dN · m) 5.62 M_(H) (dN ·m) 15.93 Tc90, minutes 19.2 Compression set, 18 200° C., 70 hours, 25%compression, %

What is claimed is:
 1. A curable composition comprising: A) afluoroelastomer comprising copolymerized units of a nitrilegroup-containing cure site monomer; and B) a hydrazide of the generalformula R¹(C(O))_(n)NHNHR², wherein said oxygen atom may optionally bereplaced with a sulfur atom and wherein n is 1 or 2; R¹ is NH₂, NHNH₂,NHR³, NR³ ₂, NHNHR², or NHC(O)NHNH₂; R² is H, CO₂R³, CH₂R⁴ or C(O)R⁴; R³is alkyl, aryl, heterocycle, or CH₂R⁴; and R⁴ is a fluoroalkyl group. 2.A curable composition of claim 1 wherein n is 1; R¹ is NH₂, NHNH₂, NHR³,NR³ ₂, NHNHR², or NHC(O)NHNH₂; and R² is H.
 3. A curable composition ofclaim 1 wherein n is 2; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHR², orNHC(O)NHNH₂; and R² is H.
 4. A curable composition of claim 1 wherein nis 1; R¹ is NH₂, NHNH₂, NHR³, NR³ ₂, NHNHR², or NHC(O)NHNH₂; and R² isCO₂R³.
 5. A curable composition of claim 1 wherein n is 2; R¹ is NH₂,NHNH₂, NHR³, NR³ ₂, NHNHR², or NHC(O)NHNH₂; and R² is CO₂R³.
 6. Acurable composition of claim 1 wherein said hydrazide is selected fromthe group consisting of carbohydrazide; 4-phenylsemicarbazide;4-phenylsemicarbazide hydrochloride; 4,4-diphenylsemicarbazide; oxamichydrazide; oxalyldihydrazide; semicarbazide; thiocarbohydrazide; andthiosemicarbazide.
 7. A curable composition of claim 1 furthercomprising a curative accelerator.
 8. A cured article made from thecomposition of claim
 1. 9. A cured article of claim 8 having a volumeswell, measured according to ASTM D1414, after exposure to 225° C. waterfor at least 168 hours of less than 5% and a compression set, 300° C.,70 hours, 15% compression, measured according to ASTM D395, of less than70%.